The present invention relates to a steel structure assembling system. The steel structure assembling system applied to assemble a movable steel beam and a fixed steel structure includes a rotating control device disposed on the movable steel beam for adjusting a relative position of the movable steel beam with respect to the fixed steel structure; an alignment target disposed on the fixed steel structure; and an image sensor disposed on the movable steel beam, sensing a target image of the alignment target and transmitting the target image to a ground operator to render the ground operator to operate the rotating control device to adjust the relative position of the movable steel beam in accordance with the acquired target image.